This invention relates to multifrequency antenna arrays, including active antenna arrays, which are useful in frequency-multiplexed multichannel communications systems.
Present-day communication satellites or spacecraft provide multichannel communication links between ground stations, often using reflector antennas with multiple feeds. Because of the weight and reliability limitations of spacecraft and performance limitations of the antennas, attention has been directed toward substitution of active array antennas for the reflector/feed systems now used. Active array antennas are well known for other uses, as described, for example, in U.S. Pat. No. 5,128,683, issued Jul. 7, 1992 in the name of Freedman et al, in which the use is air traffic control radar. A monopulse antenna array system is described in U.S. Pat. No. 5,017,927, issued May 21, 1991 in the name of Agrawal et al, which includes an active array antenna with cascaded beamformers for sum, elevation difference (.DELTA.) and azimuth .DELTA. signals. A multichannel spacecraft communication system is described in U.S. Pat. No. 5,025,485, issued Jun. 18, 1991 in the name of Csongor et al, in which plural information channels are distributed to the antennas on different phases of a common carrier frequency.
FIG. 1 is a simplified block diagram of a prior art spacecraft 10 with a communication system. The communication system includes a reflector-type uplink antenna 12 which receives a plurality of channels, which may be in non-overlapping frequency bands. The received signals are amplified in a low-noise amplifier (LNA) arrangement 14, and block converted to a lower frequency in a down-converter 16. The downconverted signals are demultiplexed in a demultiplexer 18, to separate the signals into separate channels at different carrier frequencies, which are applied to input ports 28a, 28b, 28c, . . . 28h of an antenna beamformer 30. Beamformer 30 combines the channelized signals at different frequencies applied to its input ports 28c-28h (where the hyphen represents the word "through"), to produce a plurality of output signals, one of which is on each of output ports 48(a) , 48(2) , 48(3) , . . . 48(8). The signal produced at each output port 48 of beamformer 30 of FIG. 1 includes a component at each of the different frequencies appearing at the input ports 28 of beamformer 30. The signals from output ports 48(1), 48(2), 48(3), . . . 48(8) of beamformer 30 are applied to antennas 51a, 51b, 51c, . . . 51h, respectively, of an antenna array 100. Antennas 51 as illustrated in FIG. 1 are arranged as a vertical line array, and each antenna is elongated horizontally, or includes a plurality of radiators extending orthogonal to both the array direction and the direction of radiation 49, to thereby form two-dimensional transmit array 100.
FIG. 2a is a simplified, conceptual diagram, in perspective or isometric view, of a prior-art beamformer, which may be used for transmission (or reception, if an array receiving antenna is used) in the arrangement of FIG. 1. For definiteness, the beamforming network of FIG. 2a represents transmit beamforming network 30 of FIG. 1. Elements of FIG. 2a corresponding to those of FIG. 1 are designated by like reference numerals. In FIG. 2a, eight input signal ports 28a, 28b, and 28c-28h are embodied as coaxial connectors. Each of connector ports 28a-28h receives an information channel at a different frequency, downconverted from the uplink signals received by antenna 12 of FIG. 1. The purpose of beamformer 30 is to redistribute the eight information channels among the eight antennas 51a, 51b, 51c, . . . 51h of array antenna 100, with phase selected to produce the desired beam, and amplified to compensate for system and path losses to thereby provide a predetermined signal strength at all receiving locations within the footprint. As illustrated in FIG. 2a, each input port connector 28a-28h is coupled to a corresponding power divider (PD) board 42a-42h. Each power divider board 42 divides its input signal into eight equal portions, which are made available on output signal paths 44. Each output signal path of a power divider board 42 is a coaxial signal path, designated by the same letter as the letter designation of the power divider board, together with a further numerical designation, ranging from 1 to 8, identifying its position, starting from the top of the structure. Thus, the uppermost output signal path originating from PD board 42a is designated 44a1. Other uppermost signal paths include path 44b1, originating from PD board 42b, and 44c1, originating from PD board 42c, etc. The uppermost signal path originating from PD board 44h is designated 44h1, the next lower one is 44h2. In the third layer, the output signal path from PD board 42h is 44h3. The lowermost output signal path from PD board 42h is 44h8.
The uppermost layer of signal paths 44 in FIG. 2a is coupled to an uppermost signal combining (SC) board 46(1), which combines the information signals received at different frequencies from signal paths 44a1, 44b1, 44c1 . . . 44h1, to provide a combined output signal at an uppermost coaxial output signal path 48(1). Uppermost signal path 48(1) is connected to antenna 51a of array 100 of FIG. 1. Uppermost signal combining board 46 (1) of FIG. 2a also phase-shifts the information signals as necessary, in conjunction with the phase shifts imparted by the other signal combiner boards, to cause antenna array 100 of FIG. 1 to produce a transmit beam in the desired direction. Each of the other signal combiner boards 46(2), 46(3) . . . 46(8) of FIG. 2 combines the information signals received at the eight different frequencies of the eight input signals, and makes the combined signal available on its corresponding output signal path 48(2), 48(3) . . . 48(8) (not visible in FIG. 2a), phase shifted as required.
FIG. 2b illustrates details of PD board 42 of FIG. 2a. For definiteness, PD board 42h is illustrated. In FIG. 2b, information signal at one of the input frequencies is applied from input port 28h to a cascade of three stages of 3 dB couplers, each of which divides the signal energy into two equal portions. A first stage of power division includes only 3 dB coupler 50, which divides the signal into two equal portions on signal paths 50a and 50b. The signal on path 50a is applied to a 3 dB coupler 52 of a second stage of couplers. Coupler 52 divides the signal received from path 50a into equal portions, which appear on signal paths 52a and 52b. The signal on path 52a is further divided by coupler 56 into two portions which appear at output ports 44h1 and 44h2. The operation of the remainder of PD board will be apparent from the above description. The result of the operation of PD board 44h is to produce eight identical samples of the input signal on output ports 44h1-44h8. A loss-compensating amplifier, illustrated in dash lines as 74, may be included to overcome loss of the cascade of couplers.
FIG. 2c is a simplified block diagram of a signal combiner board 46 of FIG. 2a. in FIG. 2c, uppermost signal combiner board 46(1) is illustrated in simplified schematic form. The other signal combiner boards of FIG. 2a are identical, so a description of board 46(a) suffices for all. In FIG. 2c, the information signals at the various carrier frequencies are applied to a summing circuit designated 70, and the resulting summed or combined signals are applied by way of a phase shifter 73 to the input of a power amplifier 274, which amplifies the combined signals. The amplified signals at the output of amplifier 274 are applied to output port 48(1), from which they are coupled to antenna 51a of FIG. 1. As illustrated FIG. 2c, summing circuit 70 includes a frequency multiplexing filter 72, which is well known in the art.
As described above, the prior art system receives signals by means of a reflector antenna, downconverts them, separates them according to frequency, and recombines the signals for amplification and retransmission back to Earth by means of a vertical line antenna array, which may be extended into a planar array by means of additional antennas extending horizontally fed in the same phase. This creates a directional beam in the vertical plane, which can be controlled by phase shifters in series with each antenna. Such an antenna array, while it may have gain in the orthogonal direction, cannot be independently controlled in the second plane. New satellites require higher transmitted power within their footprints. A planar or two-dimensional array may provide more power than a line array by making more ports available at which amplification can be performed, and has the additional advantage of providing controllable beamshaping in two dimensions, to allow greater concentration of the power.